It is well known that the R-T-B based permanent magnet (R represents a rare earth element, T represents Fe or Fe with part of it replaced by Co, and B represents boron) with the tetragonal compound R2T14B being its main phase has excellent magnetic properties and thus is a representative permanent magnet with high performance since it was invented in 1982 (Patent Document 1: JP S59-46008A).
The R-T-B based magnet with the rare earth element(s) R composed of Nd, Pr, Dy, Ho and/or Tb has a large magnetic anisotropy field Ha and is preferred as a material for the permanent magnet. The Nd—Fe—B based magnet with Nd being the rare earth element(s) R is widely used because it has a good balance among the saturation magnetization Is, the Curie temperature Tc and the magnetic anisotropy field Ha and is better in resource amount and corrosion resistance than R-T-B based permanent magnets using other rare earth elements R.
As a power unit used in consumer, industries and transportation equipments, the permanent magnet synchronous motor has been used. However, the permanent magnet synchronous motor in which the magnetic field generated by the permanent magnet is constant is hard to be driven as the induced voltage increases in proportion to the rotational speed. Thus, when the permanent magnet synchronous motor is operated in an intermediate/high speed region or under a low load, a field-weakening control which counterbalances the magnetic flux of the permanent magnet with the magnetic flux generated by the armature current is needed to make the induced voltage never exceed the supply voltage. As a result, a problem is there that the efficiency of the motor deteriorates.
In order to solve the technical problem mentioned above, a motor having a variable magnetic flux (hereinafter, referred to as variable magnetic flux motor) is developed which uses a magnet (a magnet with variable magnetic force), wherein in the magnet, the magnetic force changes reversibly via an action of an external magnetic field. When the variable magnetic flux motor is operated in the intermediate/high speed region or under a low load, the deterioration of efficiency in the motor due to the field-weakening effect in the prior art can be inhibited by reducing the magnetic force of the magnet with variable magnetic force.